crank falling off

t.c.claret Posts: 9
edited December 2012 in Workshop
Currently riding a bike intended for winter use with a new tiagra crankset, basically its done less than 300 miles and the non drive side crank has fallen off.
Before I rode the bike I checked that it was securely fastened to the bike. Whilst on the rides their was no creaking or any indication this would happen.
Might be a stupid question but was wondering wheather anyone knew why the crank would just fall off?


  • The short answer is that the bolts came undone. I don't know the details of tiagra cranks but most cranks now have 2 clamp bolts and a plastic bolt to apply initial tension. Some just have a bolt in the end of the axle. It is vital that these bolts are torqued up properly and then checked a few miles later. You should return to your mechanic to check that this was all done, If you didn'tdo it yourself!

    The bad news is that as the crank works loose it often wears the splines, which effectively ruins the left hand crank. Once worn you will find it difficult to tighten properly and it might keep coming loose.

    Sorry to be the bearer of bad news, hope it turns out ok.
  • thanks for the help,

    its disapointing that the company appear to have not torqued it properly, i'll have to go to their workshop. just out of interest: what does a crank fixing bolt do with regards to preventing the crank coming off? this has been lost when the crank came off.

    thanks for any help :)
  • If your axle (bottom braket) has a square end, this is tapered and the bolt pulls the crank tightly onto the taper. If your bottom bracket has splines (a series of grooves ) and one bolt in the end then the bolt pulls the crank into the splines and keps it in place. The crank will be a tight fit on the splines.
    If you have two bolts they will clamp the crank tightly onto the splines. There will also be a plastic bolt which just pulls the cranks together before you tighten up the bolts. The plastic bolt does not (indeed cannot) provide enough force to hold the crank on. it works like the top cap on your headset to provide just enough force to preload the bearings.
    Hope this helps. I have tried to explain everything simply, sorry if i have patronised you, i get the feeling you are not all that experienced.
    Check out this page for Tiagra exploded views. ... ID=jIfwFQz
    Check out a few, nobody knows the actual code numbers for Shimano parts!
  • t.c.claret wrote:
    its disapointing that the company appear to have not torqued it properly, i'll have to go to their workshop.

    As pointed out on another thread, the Shimano document on their Hollowtech II Cranksets states:

    The two left crank arm mounting bolts should be tightened alternately in stages rather than each bolt being fully tightened all at once. Use a torque wrench to check that the final tightening torques are within the range of 12 - 14 N·m. Furthermore, after riding approximately 100 km (60 miles), use a torque wrench to re-check the tightening torques. It is also important to periodically check the tightening torques. If the tightening torques are too weak or if the mounting bolts are not tightened alternately in stages, the left crank arm may come off and the bicycle may fall over, and serious injury may occur as a result.

    So there's nothing to say that the company didn't tighten the bolts correctly. However they obviously should have told you about the need to check the tightening after 60 miles and periodically thereafter.